The Five Worst Extinctions in Earth's History

Ordovician-Silurian extinction about 439 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. The toll: 25 percent of marine families and 60 percent of marine genera.

Late Devonian extinction about 364 million years ago, cause unknown. It killed 22 percent of marine families and 57 percent of marine genera. Little is known about land organisms at the time.

Permian-Triassic extinction about 251 million years ago. Many scientists suspect a comet or asteroid impact, although direct evidence has not been found. Others believe the cause was flood volcanism from the Siberian Traps and related loss of oxygen in the seas. Still others believe the impact triggered the volcanism and also may have done so during the Cretaceous-Tertiary extinction.

Permian-Triassic extinction The Permian-Triassic catastrophe was Earths worst mass extinction, killing 95 percent of all species, 53 percent of marine families, 84 percent of marine genera and an estimated 70 percent of land species such as plants, insects and vertebrate animals.

End Triassic extinction roughly 199 million to 214 million years ago, most likely caused by massive floods of lava erupting from the central Atlantic magmatic province -- an event that triggered the opening of the Atlantic Ocean.

End Triassic extinction The volcanism may have led to deadly global warming. Rocks from the eruptions now are found in the eastern United States, eastern Brazil, North Africa and Spain. The death toll: 22 percent of marine families, 52 percent of marine genera. Vertebrate deaths are unclear.

Cretaceous-Tertiary extinction, about 65 million years ago, probably caused or aggravated by impact of several-mile-wide asteroid that created the Chicxulub crater now hidden on the Yucatan Peninsula and beneath the Gulf of Mexico.

Cretaceous-Tertiary extinction Some argue for other causes, including gradual climate change or flood-like volcanic eruptions of basalt lava from Indias Deccan Traps. The extinction killed 16 percent of marine families, 47 percent of marine genera (the classification above species) and 18 percent of land vertebrate families, including the dinosaurs.